The present invention relates to novel chemical compositions comprising bridged clay and organic compounds. These compositions form materials known as nanocomposites, which possess improved mechanical and thermal properties.
Such nanocomposites can advantageously be used, for example, as cable insulation materials. PVC, used until now to sheath and insulate cables, must be replaced because of the toxic and corrosive products that can be released during combustion. However, the currently available non-halogenated fire-resistant materials are both expensive and have low resistance to heat and oil. Thus, a material is sought that is compatible with the environment and that does not have the disadvantages cited above, to replace PVC in insulating parts or in cable sheaths.
A further solution for rendering cable sheaths more fire-resistant is to add a large quantity of metal hydroxides. However, the mechanical and electrical properties of the cables degrade.
Nanocomposites are composite materials comprising sub-micronic particles dispersed in an organic matrix. In particular, lamellar inorganic materials such as graphite or silicates have the ability to intercalate organic compounds such as polymers between their lamellae. When the repulsive forces between the atoms of the organic compound exceed the attractive forces between the lamellae, the lamellar material delaminates, resulting in a hybrid structure in which the lamellae are dispersed through the organic compound matrix.
However, preparing nanocomposites generally necessitates pre-treating the lamellar material. In order for the organic compound, in general a polymer, to be able to penetrate between the lamellae, the lamellar material preferably presents an organophilic nature, which it generally does not normally possess. In that case, the surface of the inorganic material has to be pre-treated to endow it with that more organophilic nature.
International patent application WO-A-93/04117 describes nanocomposites obtained by treating the lamellar inorganic material with swelling/compatibilizing agents such as primary and secondary amines or quaternary phosphonium cation complexes with residues containing a certain number of aliphatic carbon atoms. Such long carbon chain compounds interact favorably with the intercalating compound. However, it has been shown that the thermal stability of clays treated with quaternary ammonium salts is lower. Further, ammonium salt decomposition can cause decolorization, the formation of gaseous products, and it can degrade mechanical properties.
The invention thus aims to overcome the problem of providing a nanocomposite in which the structure of the lamellar inorganic material has a higher thermal stability.
In the invention, the solution consists of using a clay bridged with a metal compound as the lamellar material. It has been shown that such bridged clays have a particularly stable structure under heat stress. The water present between the layers and bound to the cations initially present in the interlamellar spaces is expelled by the bridging that is produced by a cation exchange mechanism by a metal oxyhydroxycation and which prevents the lamellae from closing up, resulting in a higher permanent mesoporosity. This has the effect of facilitating intercalation of the organic compound and facilitating nanocomposite formation.
Finally, the presence of different metallic species can further improve the thermal stability and fire resistance. These thermal and mechanical properties are combined with a reduced weight due to the small proportion of filler compared with normal compositions. This means that nanocomposites are perfectly suited to protecting and thermally insulating articles where their weight has to be limited, such as cables.
In the invention, the nanocomposite uses a bridged clay with a lamellar structure that, after optional specific prior heat treatment, can intercalate an organic compound between its lamellae.
The nanocomposite of the invention is obtained from a compound bridged with a metal compound and an organic compound, preferably a polymer. The bridged clay acts as a filler and can be obtained by treating a natural or synthetic clay.
Certain of the lamellar clays used are also known as smectites. In the invention, the clay is preferably selected from smectites: montmorillonite, laponite, beidellite, nontronite, saponite, hectorite; and also from other clays such as kaolinite, vermiculite and sepiolite, or one of their synthetic or naturally interstratified mixtures.
The organic matrix can be a polymer, oligomer, or monomer, preferably a polymer. It may be a particle that can be transformed when molten or in the liquid state. As an example, the following can be used: polyethylene, polypropylene, and their copolymers, halogenated and non-halogenated elastomers, thermoplastic elastomers, silicones, or a mixture of such polymers, preferably polyethylene. Ethylene copolymers that can be selected include ethylene-vinyl acetate copolymers, ethylene-propylene copolymers, ethylene-alkyl acrylate copolymers, ethylene-acrylic acid copolymers, ethylene terpolymers, or said polymers containing specific groups (acids, epoxy groups, . . . ).
Polymers that can be used in the liquid state include polymers selected from polyester resins, epoxy resins, polyamides, polyimides, polyetherimides, polyamide imides, polyurethanes, or a mixture of said polymers.
The starting clay is treated with a solution of a salt of a metallic compound, preferably a solution of an iron and/or aluminum salt. After drying and heat treatment, a bridged clay is obtained.
The bridged clay can then undergo a specific treatment to render it more organophilic. To this end, it is treated with a surfactant solution, for example a quaternary ammonium salt.
The bridged clay is then mixed with the organic compound. Mixing is carried out in a flow mixer or batch mixer in the presence of 0.5% to 20% of treated clay and at a temperature in the range 80xc2x0 C. to 250xc2x0 C., more generally in the range 120xc2x0 C. to 220xc2x0 C., for a period in the range 2 minutes to 2 hours, more particularly in the range 4 minutes to 30 minutes. In the case of a polymer, the nanocomposite can be obtained, for example, by mixing with a molten polymer. This process is also known as melt intercalation. However, it is also possible to carry out in situ polymerization.
This process can produce nanocomposites with thermal properties that are improved over those of nanocomposites obtained using the conventional process.
More particularly, the invention provides a nanocomposite comprising clay and an organic compound, in which the clay is a clay bridged with a metal compound. The metal compound is preferably a metal oxide. It may comprise a proportion of another metal compound such as a metal hydroxide. Preferably, the clay is bridged by an iron and/or aluminum compound.
The nanocomposite preferably comprises a clay selected from montmorillonite, laponite, beidellite, nontronite, saponite, sauconite, hectorite, stevensite, kaolinite, halloysite, vermiculite, and sepiolite, or one of their synthetic or naturally interstratified mixtures. Laponite and montmorillonite are particularly preferred.
Preferably, the organic compound in the nanocomposite is a polymer. In one embodiment, the polymer is preferably selected from polyethylene, polypropylene, ethylene copolymers, non-halogenated elastomers, thermoplastic elastomers, silicones, or mixtures thereof. In a further embodiment, the polymer is selected from polyester resins, epoxy resins, polyamides, polyimides, polyetherimides, polyamide imides, polyurethanes, and mixtures thereof.
In one aspect of the invention, an advantageous application of these nanocomposites is cable insulation. The term xe2x80x9ccablexe2x80x9d means bundles of conductive wires or fiber optics protected by insulating sheaths used to supply electricity or in telecommunication networks. Preferably, the nanocomposites are used in insulating telecommunication cables and power cables.
The invention thus concerns a power cable the sheath of which comprises a nanocomposite of the invention. The invention also concerns a telecommunications cable the sheath of which comprises a nanocomposite. Preferably, the sheath is constituted by a nanocomposite. In a further embodiment, the cable is provided with an outer coating comprising a nanocomposite.
The invention also concerns a process for producing said nanocomposite, comprising the steps of preparing a clay bridged by a metal compound and mixing with an organic compound.
In one embodiment, preparing the bridged clay comprises the steps of adding a mixture of an oligomeric solution of a metal compound to the clay in suspension, eliminating the excess solution by centrifuging, washing the residue, drying, and heat treatment.
In one embodiment, the bridged clay is treated with a compatibilizing compound prior to mixing with the organic compound. Preferably, the compatibilizing compound is a quaternary ammonium salt.